Electric lamp mount having a beaded filament coil

ABSTRACT

The coiled tungstem filament of an electric incandescent lamp is provided with a fused bead of tungsten-iron alloy at each end and is so fastened to the lead wires of the mount assembly that the beads are located outboard from and in contiguous relationship with the respective lead wires. The beads are larger than the diameter of the filament coil and spaced a predetermined distance apart and thus serve as built-in guides or &#39;&#39;&#39;&#39;reference points&#39;&#39;&#39;&#39; during the filament-mounting operation which accurately control the lighted-length of the mounted coils.

United States Patent 91 Petro et al.

ELECTRIC LAMP MOUNT HAVING A BEADED FILAMENT COIL Inventors: James Petro, Little Falls; Clair M. Rively, Old Bridge, both of NJ.

Assignee: Westinghouse Electric Corporation,

Pittsburgh, Pa.

Filed: July 12, 1972 Appl. No.: 271,103

Related US. Application Data Division of Ser. No. 163,651, July 19, 1971, which is a division of Ser. No. 792,988, Jan. 22, 1969.

US. Cl. ..3l3/344, 313/271, 313/345 Int. Cl ..H01j l/16, HOlj 19/10, HOlj 19/06 Field of Search ..3l3/27l, 344, 345

[4 1 Apr. 3, 1973 [56] References Cited UNITED STATES PATENTS 3,504,218 3/1970 Emidy ..3l3/344 Primary Examiner-Ronald L. Wibert Assistant ExaminerPaul A. Sacher AttorneyA. T. Stratton et al.

ABSTRACT The coiled tungstem filament of an electric incandescent lamp is provided with a fused bead of tungsten-iron alloy at each end and is so fastened to the lead wires of the mount assembly that the beads are located outboard from and in contiguous relationship with the respective lead wires. The beads are larger than the diameter of the filament coil and spaced a predetermined distance apart and thus serve as builtin guides or reference points during the filamentmounting operation which accurately control the lighted-length of the mounted coils.

6 Claims, 21 Drawing Figures V PATENTEUAPRB ms SHEET 2 UP 3 PATENH-IUAPR3 1975 3,725,720

SHEET 3 OF 3 ELECTRIC LAMP MOUNT HAVING A BEADED FILAMENT COIL CROSS-REFERENCES TO RELATED APPLICATIONS BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to electric lamps and has particular reference to an improved filament coil and mount assembly for an incandescent or fluorescent lamp.

2. Description of the Prior Art As is well-known, filamentary coils for fluorescent and incandescent lamps are manufactured by winding a tungsten wire around a mandrel of dissimilar metal, such as iron, mechanically cutting the resulting composite wire into segments of the desired length, and chemically dissolving the mandrel to provide the desired individual coils of tungsten wire. These coils are then clamped (or otherwise fastened) to the ends of the lead wires of the mount assembly which is then sealed into the lamp envelope. In the case of a fluorescent lamp the turns of the coils are coated with electron-emission material and serve as cathodes. When used in incandescent lamps, the coils must be clamped to the leads so that a carefully controlled number of coil turns are disposed between the leads and the lamp will thus operate at the proper wattage rating.

It has been found that when the coils are cut during manufacture burrs are formed on the severed ends of the finished coils which tend to snag and become entangled with the turns of other coils. This tangling problem made it very difficult to design a satisfactory coil feeder which will automatically separate and supply the coils to a mounting machine. The prior art coils, accordingly, had to be manually separated and fed into the mounting machine. This is a time-consuming tedious operation and materially increases the manufacturing cost of the lamps. In addition, large quantities of finished coils sometimes had to be scrapped during inspection because it was impossible to untangle them. The percent shrinkage is thus very high and further increases the lamp manufacturing cost.

The present invention provides an economical and practical solution to the aforementioned tangling problem and reduces the-manufacturing cost of electric lamps by providing filament coils which can be readily separated and fed into the. mount-making machine by an automatic coil-feeder and then be fastened to the lead wires in a uniform and precisely controlled manner.

SUMMARY OF THE INVENTION The aforesaid objectives andother advantages are achieved in accordance with this invention by providing an integral nodule or bead of fused ductile metal at each end of the filament coils. Briefly, the ends of the iron mnmlnl are melted in nitu nnd't'urm pools of mol ten iron which dissolve the overlying tungsten wire turns, thus producing integral beads of tungsten-iron wire alloy that are fused to and merge with the respective end turns of tungsten wire. The beads are formed in such a manner that the coil turns remain in their original unrecrystallized state. Since the beads contain tungsten, they are not dissolved by the acid used to dissolve and remove the iron mandrel so that the finished coils are terminated by ductile beads which close the respective end turns of the coil and merge with and enclose the cut ends of the tungsten wire. The finished coils can thus be processed, inspected and shipped en masse without becoming tangled or fracturing. Shrinkage during manufacture and handling is drastically reduced and the coils readily separate for automatic feeding into the mount-making machine.

Various methods for forming such tungsteniron alloy beads on the ends of finished refractory wire coils or as integral parts of leg inserts for a filament coil-are also disclosed.

The aforesaid beaded coils provide an improved filament mount assembly which has a filament whose lighted length is precisely controlled. This is achieved by using the enlarged integral beads on the ends of the coil as reference points when the lead wires are secured to the coil legs during the filament-mounting operation.

BRIEF DESCRIPTION OF THE DRAWINGS the composite wire used in making the coil shown in the preceding Figures;

FIG. 5 is an enlarged perspective view of a portion of the aforementioned composite wire after it has been wound around the iron mandrel to form a continuous stock wire;

FIG. 6 is an enlarged elevational view of a severed segment of stock wire after the beads have been formed at each end and before the iron mandrel and iron filler wire have been dissolved;

FIGS. 7a to 7d are elevational views of a section of stock wire illustrating the various steps in concurrently beading and severing an embryonic coil segment from the end ofa continuous supply of stock wire;

FIG. 8 is a similar view of-one end of the finished non-recrystallized tungsten wire electrode coil after the iron mandrel and filler wire have been removed from i FIGS. 11a to 11d are elevational views illustrating various operations in another method of forming beads on filament coils wherein short pieces of metal wire are inserted into the ends of a previously formed coil and then melted with lasers to form the desired integral fused beads;

FIG. 12 is a similar view depicting another beadforming method wherein the end of a metal wire is being melted and fused with the endturn of a preformed coil as the wire is being fed into the coil;

FIG. 13 is a perspective view illustrating a method for forming beads as integral parts of leg inserts of a preformed filament coil; and

FIGS. 14a and 14b are enlarged elevational views of a beaded coiled-coil filament and the associated lead wire portions of a mount assembly for an incandescent lamp, respectively, illustrating the manner in which enlarged beads are used to control the lighted length of the mounted filament pursuant to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS (FIGS. 1-6) In FIG. 1 there is shown a mount assembly for a fluorescent lamp. The mount 10 consists of the usual glass stem 11 that has a flare 12 at one end and an axially depending glass exhaust tube 13 which extends into the stem tube and, together with an aperture 14 formed in the side wall of the tube, provides a passageway for evacuating and mercury-dosing the lamp after the stem 11 has been sealed into the lamp envelope. A pair of lead wires 16, 17 are hermetically sealed through a press formed on the end of the stem 11 and the ends of the wires are formed into clamps 18. These clamps are closed around the ends of a thermionic cathode consisting of a barrelless electrode coil 20 of nonrecrystallized tungsten wire that is coated with a suitable electron-emission material B, such as the wellknown mixture of alkaline earth oxides.

As will be noted, each end of the coil 20 is provided with a nodule such as a generally spherical bead 21 of fused ductile metal that extends across the end face of the coil. It will also be noted that the coating E of emission material is confined to the medial portion of the coil 20 and that the coil turns immediately adjacent the clamps 18 are uncoated. Such coils are referred to in the art as barrelless coils because they consist of a plurality of spaced turns that are of the same diameter and thus form a helix that is of linear configuration and the same cross-sectional dimension throughout its length. Such barrelless coils, accordingly, do not have the enlarged secondary turns and medial coil barrel portion characteristic of coiled-coil or triple-coiled filaments.

As shown in FIGS. 1 and 2, the beads 21 are integral with and terminate the end turns 22 of the coil 20 and are of approximately the same size as the outer diameter of the coil. The beads 21 accordingly merge with and close the ends of the coil 20.

As illustrated in the enlarged view of the coil 20 shown in FIG. 3, the coil consists of a coiled core wire 23 of suitable refractory material (non-recrystallized tungsten for example) which has a winding of fine refractory wire 24, such as non-recrystallized tungsten, loosely coiled therearound. The turns of the fine wire 24 enclose the core wire 23 and form a basket-like structure or matrix which increases the emission-holding capacity of the coil 20. When the medial portion of the coil 20 is coated with the emission material E after the coil is attached to the lead wires l6, 17, the emission material E fills the matrix formed by the loose overwinding of fine wire 24 and bridges the turns 22 of the coil 20, as is shown in FIG. 1.

The electrode coil 20 is manufactured by pairing the tungsten core wire 23 with a slightly larger filler wire 25 of dissimilar metal, such as iron, that can be subsequently chemically dissolved from the wound coil without affecting the tungsten core wire. The paired tungsten core wire 23 and iron filler wire 25 constitute a dual-strand core component. The fine tungsten wire 24 is then tightly wound around the paired core wire 23 and filler wire 25 (that is, the aforesaid dual-strand core component) to form the composite wire 26 shown in FIG. 4. This composite wire 26 is, in turn, wound around an iron mandrel 27 at the required TPI to form a continuous coil-mandrel composite 28 shown in FIG. 5. For convenience, this composite 28 is referred t herein as the stock wire.

In the prior art, after the stock wire 28 was mechanically cut into segments of the desired lengths the resulting segments were placed in an acid bath (e.g., hydrochloric acid) which dissolved the iron filler wire 25 and iron mandrel 27 and thus produced a finished coil consisting of the coiled tungsten core wire 23 and loose overwinding of fine tungsten wire 24. Since the tungsten core wire 25 is of such small diameter, it is impossible as a practical matter to mechanically cut it cleanly. As a result, burrs were unavoidably left on the cut ends. Because the core wire 25 is only loosely enclosed by the fine wire winding 24, the burred ends of the core wire naturally protruded from the ends of the finished coils and created the aforementioned snagging and tangling problem when the coils were placed into a hopper and handled en masse.

This snagging and tangling problem is solved by melting the ends of the iron mandrel 27, before the latter is chemically removed, and thereby forming an integral nodule or bead 21 of fused ductile tungsten-iron alloy at each end of the segments of stock wire 28. Since the acid that is used to dissolve the iron mandrel 27 and filler wire 25 does not attack tungsten, these tungsteniron alloy beads 21 remain in place on the end turns of the finished coil 20, as shown in FIGS. 1 and 2, after the iron components have been removed.

When the iron mandrel 27 is melted the resultant pool of molten iron dissolves the overlying tungsten components of the composite wire 26 so that the ends of the core wire 25 and overwound fine wire 24 merge with and are anchored in the respective beads 21. The end burns of the finished coil 20 are thus terminated by globular beads 21 that are substantially smooth and larger than the spacing between the coil turns 22, thereby completely eliminating the troublesome burred ends and interlocking problem characteristic of the prior art coils. The beaded-end coils 20 can thus be processed and shipped in bulk without the coils snagging one another and becoming entangled. As a result, they can be readily separated and supplied to a mounting machine by a suitably designed automatic coil-feeder.

While various concentrated and controllable heat sources, such as a focused electron beam, a plasma torch or a sharply defined oxy-hydrogen flame, can be used to melt the ends of the iron mandrel 27, a laser beam is preferred since it can be readily focused with great accuracy onto the ends of the iron mandrel.

A segment 29 of non-recrystallized stock wire 28 that has enlarged integral beads 21 of ductile tungsteniron alloy formed on each end by means of a laser beam in accordance with the present invention is shown in FIG. 6. As will be noted, the beads 21 merge and are integral with the ends of the composite wire 26 that is wound about the iron mandrel 27. Such segments 29 are of precisely controlled length L and are, in effect, embryonic filamentary coils in that they need only be immersed in an acid bath, washed and dried to be transformed into finished coils.

BEADING AND SEVERING OPERATION (FIGS. 7-8) The beading and severing of the stock wire 28 into segments 29 is preferably achieved simultaneously by a single operation.

As shown in FIG. 7a, the stock wire 28 is first subjected to a laser beam 30 which is focused onto the axis of the iron mandrel 27 at a point located the desired distance L from the end of the head 21 formed on the free end of the stock wire by the previous beading severing operation. The intense heat generated by the impinging laser beam 30 rapidly melts the iron mandrel 27 and forms a pool of boiling iron which melts the overlying portion of the iron filler wire and the corresponding portions of the tungsten core and fine wires 23, 24 that constitute the composite wire 26. A molten pool 21' of tungsten-iron alloy is thus formed by the impinging laser beam 30, as shown in FIG. 7b. When this occurs an axial pull, indicated by the arrow in FIG. 7c, is applied to the free end of the stock wire 28 which causes the molten pool 21 to begin to subdivide into two globular masses. The axial pull or force is maintained until the molten pool of tungsten-iron alloy completely separates. The laser is then turned off and, due to the surface tension of the molten alloy remaining on the severed ends of the stock wire 28, the respective globular masses of molten alloy inherently take the shape of generally spherical beads 21 which are integral and merge with the severed ends of the composite wire 26, as shown in FIG. 7d.

The molten globules of alloy rapidly solidify, thus providing fused tungsten-iron beads 21 on the end of the free end of the stock wire 28 and on the proximate end of the newly formed segment 29 which has just been severed. The beaded segment 29 (depicted at the right in FIG. 7d) is accordingly identical with the segment 29 shown in FIG. 6 and is of predetermined length L.

As illustrated in FIG. 8, the beads 21 merge and are fused with the ends of the tungsten core wire 23 and overwinding of fine tungsten wire 24 in the finished coil 20. The beads 21 thus firmly anchor the tungsten wires in place and provide a smooth rounded closure at each end of the finished coil 20 which is too large to fit between the turns 22 of other such coils and thus inherently prevents coil tangling.

After the severed-beaded segment 29 is withdrawn and deposited in a hopper, the stock wire 28 is advanced a distance L relative to the laser and the operation just described is repeated.

It should be noted that the laser beam 30 does not actually cut the stock wire 28 in the strict sense of the term but merely melts the iron mandrel 27 and forms a pool of boiling iron which then dissolves the overlying portions of the composite wire 26 to form a bulbous mass of molten tungsten-wire alloy. The severing of the stock wire 28 is thus actually accomplished by the axial pull exerted on the free end of the stock wire after the molten mass has been formed. This is important since.

the temperature of the iron mandrel 27 adjacent the molten pool of tungsten-iron alloy is too low to effect recrystallization of the tungsten in the time it takes to achieve the melting and severing operations. If this were not the case, then the unmelted portions of the tungsten wires 23, 24 would be recrystallized and become brittle with the result that the beaded end turns would fracture and separate from the finished coil 20 unless the latter were very carefully handled.

Preliminary test data indicates that the temperature of the iron mandrel 27 adjacent the molten pool 21 of tungsten-iron alloy is approximately 1,400C whereas the recrystallization temperature of tungsten is about l900C. Iron has a melting point of approximately 1535 C. Thus, both the beads 21 and the tungsten wires comprising the turns 22 of the finished coil 20 are ductile and in an unrecrystallized state.

Analysis of fused tungsten-iron beads formed on the ends of barrelless tungsten electrode coils 20 of the type described shown that the beads consisted of Fe, 20% W and approximately 5% Fe w (weight percent). On the basis ofa published 38% Fe-W phase diagram by Hansen, it is theorized that approximately only 4.5 percent (by weight) of the tungsten was in solid solution with the iron and thus constituted a true tungsten-iron alloy. The remainder of the tungsten and iron was not alloyed and comprised admixed metal in the form of a two-phase Fe-W casting or body. The term tungsten iron alloy as used herein and in the claims accordingly covers an admixture of Fe and W that is fused but which may or may not contain a true solid solution or alloy of Fe-W.

SPECIFIC EXAMPLE BARRELLESS COIL As a specific example of the various values and parameters for those who wish to practice the invention, the barrelless 40 watt fluorescent electrode coil 20 of the type illustrated and described above has an overall length of approximately eleven sixteenths inch 17 mm.). The diameter of the iron mandrel 27 is approximately 0.016 inch, the diameter of the iron filler wire 25 is approximately 0.005 inch, the diameter of the tungsten core wire 23 is approximately 0.0023 inch, the diameter of the overwound tungsten wire 24 is approximately 0.001 inch, and the diameter of the finished coil was approximately 0.03. inch. A watt CO laser was employed and its beam was focused into a spot on the iron mandrel that was approximately 0.005 inch in diameter. The wavelength of the radiation produced by the laser was 10.6 microns. The power density of the focused laser beam which impinged upon the iron mandrel was approximately 4 million watts per sq. inch. The laser was energized for approximately 0.07 second and a pull of approximately 5 ounces was applied to the free end of the stock wire 28 to sever the molten pool of tungsten-iron alloy. The entire beading and severing operation was completed within the time that the laser was energized, that is, within 0.07 second.

Removal of the iron mandrel 27 and iron filler wire 25 from the beaded segments 29 of stock wire 28 was accomplished by immersing the segments in concentrated hydrochloric acid for approximately 30 minutes and the resulting coils were then washed in deionized water and in alcohol and dried.

ALTERNATIVE BEADING METHODS (FIGS. 9-13) In FIG. 9 there is shown an alternative method for severing a continuous length of stock wire 28 into a plurality of segments of predetermined length L by means of a laser-beading operation and a subsequent separate cutting operation. As shown, the stock wire 28 is indexed in a predetermined manner past a laser 60 which is energized in timed sequence with the index speed so that the stock wire 28 is melted ata plurality of uniformly spaced points to provide a series of fused tungsten-iron alloy nodules or beads 92. These beads 92 are subsequently mechanically severed by a knife 93 to provide individual segments of predetermined length L. In contrast to the segments previously described, the segments formed in accordance with this embodiment are terminated at each end by a bisected globular bead 94 that have substantially flat end faces. In the finished coil, these bisected beads 94 extend tranversely of the coil axis and are integral with and terminate the respective end turns of the coil.

In FIG. 10 there is shown another method of providing fused metallic beads on the ends of a refractory wire coil. In this embodiment the sequence of operations is reversed from that shown in FIG. 9. As illustrated, the stock wire 28 is first cut into individual segments 29' by a pair of cutting knives 95 that are so spaced that the segments are initially longer than the desired finished length L and sections of a length Y" protrude from each end of the cut segments. The protruding sections are of such length that, when melted, they will form tungsten-iron beads of the desired size. In the case of the 40 watt barrelless electrode coils described above, the protruding sections are each approximately one half mm. in length.

The precut segments 29' of wire stock 28 are aligned with one another and indexed, as by a suitable conveyor, past a pair of lasers 60 that are so spaced and energized that their beams 30 strike and melt the protruding sections of the respective segments 29' as they are indexed into aligned position with the lasers. The resulting pools 21 of molten alloy inherently form globular beads 21 of such size that the finished segments 29 are of the desired length L. As will be obvious, the foregoing sequence of operations can be automated by providing suitable means for actuating the knives 95 in times sequence with the index of the conveyor and the operation of the lasers 60.

A method for making beaded tungsten electrode coils (or filaments) which require a heat-treating operation to set the coil on the mandrel is shown in FIGS. 11a through 11d. According to this method, the filler wire and mandrel are composed of a dissimilar refractory metal, such as molybdenum, which will withstand the heat-treating temperature. The resulting heat-treated stock wire is mechanically cut into segments of predetermined length and the refractory mandrel and filler wire are chemically dissolved in the regular fashion to provide a tungsten coil 20 (FIG. 11a) that is identical to the prior art coils in that it consists of a tungsten core wire 23 and a loose overwinding of fine tungsten wire 24. However, the coil 20 is slightly longer than the desired finished length.

As shown in FIG. 11b, small rod-like sections 96 of iron wire are inserted into each end of the coil 20' so that the ends of the respective wire sections are substantially flush with the end turns of the coil. The ends of the coil 20' are then aligned with a pair of lasers that are so spaced that the focused laser beams 30 strike the inserted iron wire sections at preselected points located inwardly from the ends of the coil (as shown in FIG. 11c). The spacing between the lasers 60 is such that the finished coil 20 (FIG. 11d) has the desired integral beads 21 of tungsten-iron alloy formed at each of its ends and the coil is of the desired predetermined length L. This method thus enables beaded tungsten wire coils to be made with refractory metal mandrels which have a melting point too high to permit them to be melted into beads without recrystallizing the tungsten wire coil.

Instead of inserting precut lengths of iron wire into the ends of a mandreless tungsten wire coil 20 as illustrated in FIG. 11, a wire 97 composed of iron (or other suitable metal) can be fed axially into one end of the coil 20' and concurrently melted by a laser beam 30 to form a molten ball 21 of iron that is located within the end turn of the coil and dissolves the latter to form an integral bead of molten tungsten-iron alloy. After the end turn of the coil 20' has been dissolved the laser 60 is deenergized and the resulting head is allowed to cool. The iron wire 97 is then severed flush with the outer end of the bead either by the laser 60 or by a knife. The iron wire 97 can also be fed into the coil 20' between its turns from a position located to one side rather than at the end of the coil.

An electrode or a filament coil of tungsten wire can also be provided with a fused metal bead at each end which constitutes an integral part of a coil leg insert. Such a coil is shown in FIG. 13 and is made by inserting sections 98 of iron wire or rod into each end of a performed tungsten wire coil 99' and melting back the ends of the tungsten coil and iron wire sections to form pools 21' molten tungsten-iron alloy (one of which is shown in FIG. 13a). This process is thus quite similar to that shown in FIG. 11 except that the sections 98 of iron wire are longer and only the outer ends thereof are melted by the laser beam 30. Hence, the finished coil 99 (FIG. 13b) is provided at each end with a fused tungsten-iron bead 21 which constitutes the end of the unmelted portion of the respective iron wires 98, and the unmelted wire portions enclosed by the coil turns serve as leg inserts.

When the finished coil 99 is mounted on its lead wires, the latter are fastened either by spot welding or clamping to the end portions of the coil that contain the iron wire inserts 98. While the medial portion of the coil 99 is here shown as comprising a plurality of uniformly spaced primary turns of the same diameter, it within the scope of the present invention that such medial portion can comprise a plurality of largerdiameter secondary turns of a coiled-coil filament of the type used in incandescent lamps and as heavy-duty electrodes in high-output fluorescent lamps.

INCANDESCENT LAMP MOUNT EMBODIMENT (FIG. 14)

The terminating beads of fused ductile metal can also be advantageously employed to control the effective lighted length of an incandescent lamp filament and this embodiment is shown in FIG. 14. As is illustrated in FIG. 14a, the filament 100 is of coiled-coil construction and thus has a barrel portion comprising a plurality of spaced secondary turns 101 which is terminated by longitudinally extending coil legs 102. Each of these legs are provided with beads 21 of fused tungsten-iron alloy. As shown, the beads 21 are larger than the outer diameter of the coil legs 102 and are spaced a predetermined distance apart.

As shown in FIG. 14b, the coiled-coil filament 100 is mounted on the lead wires 103 of the mount assembly in such a manner that the latter are contiguous to and preferably abut against the inwardly disposed surfaces of the respective beads. Since the beads 21 are spaced a precise distance apart, they serve as guides or reference points during the clamping operation and provide a very economical and reliable means for accurately controlling the length of the filament 100 which is suspended between the lead wires 103. The effective lighted length of the mounted filament 100 can thus be maintained within very tight tolerances.

We claim as our invention:

1. A mount assembly for an electric lamp or similar device comprising:

a vitreous stern,

a pair of spaced lead-in conductors that are sealed through a portion of and extend beyond said stem, and

a filament of coiled refractory metal wire that is terminated at each end by a longitudinally extending leg, each of said legs having an integral nodule of fused metal that laterally protrudes beyond the respective legs,

said lead-in conductors being attached to the portions of the filament legs that are contiguous with and located inwardly from the respective nodules so that the part of said filament that is suspended between said conductors is of predetermined length.

2. The mount assembly of claim 1 wherein;

said filament legs comprise a plurality of spaced turns that are formed from said refractory metal wire, and

said nodules of fused metal constitute the ends of said filament legs.

3. The mount assembly of claim 2 wherein;

the medial part of said coiled filament that is suspended between said lead-in conductors comprises a coiled-coil helix having a plurality of spaced secondary turns,

the legs of said filament are substantially straight and comprise a plurality of spaced primary turns,

said nodules comprise globular beads of fused ductile metal, and

said lead-in conductors are in substantially abutting relationship with the inwardly disposed surfaces of said beads.

4. The mount assembly of claim 2 wherein;

said nodules comprise the ends of metal wire members that extend predetermined distances into the filament legs and thus comprise leg inserts, and said lead-m conductors comprise wires that are fastened to the portions of said filament legs that contain the leg inserts.

5. The mount assembly of claim 2 wherein;

said coiled filament is of linear configuration and comprises a plurality of spaced turns that are substantially uniform in diameter.

said filament is composed of tungsten wire, and

said nodules are at least partially composed of tungsten-iron alloy.

6. The mount assembly of claim 5 wherein;

the turns of said coiled filament have a loose overwinding of fine refractory metal wire thereon,

said nodules comprise globular beads that merge with and anchor the respective ends of said fine wire overwinding, and

at least the medial portion of said coiled filament that is suspended between said lead-in conductors is coated with an electron-emissive material that fills the coil turns and the matrix formed by said loose overwinding of fine wire. 

2. The mount assembly of claim 1 wherein; said filament legs comprise a plurality of spaced turns that are formed from said refractory metal wire, and said nodules of fused metal constitute the ends of said filament legs.
 3. The mount assembly of claim 2 wherein; the medial part of said coiled filament that is suspended between said lead-in conductors comprises a coiled-coil helix having a plurality of spaced secondary turns, the legs of said filament are substantially straight and comprise a plurality of spaced primary turns, said nodules comprise globular beads of fused ductile metal, and said lead-in conductors are in substantially abutting relationship with the inwardly disposed surfaces of said beads.
 4. The mount assembly of claim 2 wherein; said nodules comprise the ends of metal wire members that extend predetermined distances into the filament legs and thus comprise leg inserts, and said lead-in conductors comprise wires that are fastened to the portions of said filament legs that contain the leg inserts.
 5. The mount assembly of claim 2 wherein; said coiled filament is of linear configuration and comprises a plurality of spaced turns that are substantially uniform in diameter. said filament is composed of tungsten wire, and said nodules are at least partially composed of tungsten-iron alloy.
 6. The mount assembly of claim 5 wherein; the turns of said coiled filament have a loose overwinding of fine refractory metal wire thereon, said nodules comprise globular beads that merge with and anchor the respective ends of said fine wire overwinding, and at least the medial portion of said coiled filament that is suspended between said lead-in conductors is coated with an electron-emissive material that fills the coil turns and the matrix formed by said loose overwinding of fine wire. 